1. Field of the Invention
The present invention relates to a switching power source apparatus that is simple and low cost.
2. Description of Related Art
FIG. 1 is a circuit diagram illustrating a switching power source apparatus according to a related art. This switching power source apparatus is a current resonant switching power source apparatus that receives a DC input voltage Vin generated by, for example, rectifying and smoothing a commercial AC voltage and supplied from a DC power source Vin. Both ends of the DC power source Vin are connected to a series circuit that includes first and second switching elements Q11 and Q12 are MOSFETs.
Connected between the drain and source of the switching element Q12 (or Q11) are a voltage resonant capacitor Cv1 and a first resonant circuit that includes a resonant reactor Lr1, a primary winding Np1 of a transformer T1, and a current resonant capacitor Ci1. The resonant reactor Lr1 may be a leakage inductance of the transformer T1.
A diode D1 is connected between the drain and source of the switching element Q12 and a diode D2 is connected between the drain and source of the switching element Q11. The diodes D1 and D2 may be parasitic diodes of the switching elements Q12 and Q11, respectively.
On the secondary side of the transformer T1, secondary windings Ns11 and Ns12 are wound in opposite phase and are connected in series. Voltages generated by the secondary windings Ns11 and Ns12 are rectified by diodes D11 and D12 and are smoothed by an output smoothing capacitor Co1 into an output voltage Vo1.
A controller 10 alternately provides the gates of the switching elements Q11 and Q12 with gate signals that have the same ON width and contain a dead time to prevent the switching elements Q11 and Q12 from simultaneously turning on.
In response to the gate signals, the switching elements Q11 and Q12 alternately turn on/off, to pass resonant currents Q11i and Q12i as illustrated in FIG. 2. This results inpassing sinusoidal resonant currents D11i and D12i through the diodes D11 and D12 on the secondary side of the transformer T1.
The output voltage Vo1 is fed back through an insulating device such as a photocoupler (not illustrated) to the controller 10 on the primary side. According to the fed-back signal, the controller 10 controls the switching frequency of the switching elements Q11 and Q12 in such a way as to maintain the output voltage Vo1 at a predetermined value.
According to this related art, a current passes in a negative direction (a forward voltage of the diode D2 (D1)) through the diode D2 (D1) when the switching element Q11 (Q12) is ON as illustrated in FIG. 2, to cause no switching loss. Due to resonance, no surge voltage occurs in an OFF state of the switching element Q11 (Q12). Accordingly, the switching elements Q11 and Q12 may have a low withstand voltage to improve the efficiency of the apparatus.
The current resonant switching power source apparatus of FIG. 1, however, alternately causes the sinusoidal currents D11i and D12i on the secondary side, and therefore, the currents D11i and D12i demonstrate discontinuity. As a result, a ripple current Co1i of the output smoothing capacitor Co1 becomes about 50% to 70% of an output current, which is larger than that of a forward converter that continuously causes a current. An electrolytic capacitor usually used for the output smoothing capacitor Co1 must follow a ripple current standard. For this, the output smoothing capacitor Co1 is usually a plurality of electrolytic capacitors connected in parallel. This capacitor configuration results in increasing the cost and size of the switching power source apparatus.
To solve this problem, Japanese Unexamined Patent Application Publication No. H04-105552 (Patent Document 1) discloses a switching power source apparatus that connects a plurality of circuits in parallel and operates the circuits by shifting the phases of the circuits from one to another, thereby reducing a ripple current of electrolytic capacitors.
The related art of Patent Document 1, however, must have a circuit for dividing the frequency of a pulse signal from a high-frequency oscillator arranged in a controller, to complicate the controller and increase the cost of the apparatus.
Another related art is disclosed in Japanese Unexamined Patent Application Publication No. 2010-110114 (Patent Document 2). This is a switching power source apparatus including a first converter that has a first transformer and a series circuit of first and second switching elements, a second converter that has a second transformer and a series circuit of third and fourth switching elements, a series circuit that is connected to both ends of the second switching element and includes a primary winding of a third transformer and a third capacitor, the third transformer having first and second secondary windings wound in opposite polarity, a first resonant series circuit that is connected in series with the first secondary winding of the third transformer and includes a first resonant reactor and a first resonant capacitor, a first controller that turns on/off the third switching element according to a current of the first resonant series circuit, a second resonant series circuit that is connected in series with the second secondary winding of the third transformer and includes a second resonant reactor and a second resonant capacitor, and a second controller that turns on/off the fourth switching element according to a current of the second resonant series circuit. The first resonant series circuit causes a current having a 90-degree phase delay with respect to a voltage generated by the first secondary winding of the third transformer, and according to the current of the first resonant series circuit, the third switching element is turned on/off. The second resonant series circuit causes a current having a 90-degree phase delay with respect to a voltage generated by the second secondary winding of the third transformer, and according to the current of the second resonant series circuit, the fourth switching element is turned on/off (FIG. 9 of Patent Document 2). As a result, the second converter operates with a 90-degree phase difference with respect to the first converter. Only by adding a simple circuit, this related art realizes a phase-shifted parallel operation and reduces a ripple current of an output smoothing capacitor.